This invention relates to a process and plant for separating air.
The most important method commercially for separating air is by rectification. In such a method there are typically performed steps of compressing and purifying the air, fractionating the compressed, purified, air in the higher pressure column of a double rectification column comprising a lower pressure rectification column in addition to the higher pressure one; condensing, by indirect heat exchange with oxygen-rich fluid separated in the lower pressure column, nitrogen vapour separated in the higher pressure rectification column; employing a first stream of the resulting condensate as reflux in the higher pressure rectification column and a second stream of the resulting condensate as reflux in the lower pressure rectification column; withdrawing an oxygen-enriched liquid air stream from the higher pressure rectification column; introducing an oxygen-enriched vaporous air stream into the lower pressure rectification column, and separating the oxygen-enriched vaporous air stream therein into oxygen-rich and nitrogen-rich fractions.
Purification of the air is performed so as to remove impurities of relatively low volatility, particularly water vapour and carbon dioxide. If desired, hydrocarbons may also be removed.
In order to meet the refrigeration requirements of the air separation plant, a stream of pressurised fluid, typically either air or nitrogen, is expanded in an expansion turbine with the performance of external work. Oxygen-rich and nitrogen-rich products may be taken from the lower pressure rectification column in vapour state or liquid state, or both states. In addition, an argon-enriched oxygen stream may be withdrawn from a region of the lower pressure rectification column where the argon concentration is in the order of 10 times greater than in the incoming air and separated in a further rectification column to produce an impure or pure argon product.
In order to maintain an energy balance within the arrangement of columns, it is generally necessary to employ a net flow rate of liquid into the columns equal to the net rate at which liquid products are withdrawn therefrom. Thus, as a greater proportion of the products of the air separation are taken in liquid state from the columns, so a greater proportion of the air needs to be introduced into the arrangement of columns in liquid state. In practice, the majority of such liquid air is typically introduced into the higher pressure rectification column. The result is that the proportion of air entering the higher pressure rectification column in vapour state is reduced, and therefore less nitrogen is separated from the air in the higher pressure rectification column. For each unit of liquid nitrogen taken as product, the nitrogen separated in the higher pressure rectification column is reduced by about 0.4 units. The reflux to the lower pressure rectification column is therefore reduced by about 1.4 units (the one unit of extra liquid nitrogen product and the 0.4 units reduction in the nitrogen separated in the higher pressure rectification column). A reduction in the supply of reflux to the lower pressure rectification column also takes place if a vaporous nitrogen product is taken directly from the higher pressure rectification column.
The reduction in reflux in the lower pressure rectification column causes there to be a reduction in the yield or recovery of oxygen. If an argon product is separated, there is also a reduction in the yield or recovery of argon. The reduction in argon recovery tends to become more marked than that in the oxygen recovery as, for example, liquid nitrogen production is increased. In practice, there is therefore a ceiling placed on the proportion of the products of the air separation and particularly, for example, the proportion of liquid nitrogen that can be taken from the rectification columns in liquid state while still obtaining satisfactory yields of argon and/or oxygen. It is an aim of the present invention to provide a process and plant which enable the ceiling to be raised.